Transmission Media

EE4220 Communications system

Media

• Basic function of media – carry flow of information in form of bits through a LAN• In a copper based network, bits will be electrical signals• In a fiber based network, bits will be light pulses

• Physical path between transmitter and receiver• Two Types: Wired and Wireless• Characteristics and quality of data transmission are determined

by characteristics of medium and signal

The name 10BASE5 is derived from several characteristics of the physical medium. The 10 refers to its transmission speed of 10 Mbit/s. The BASE is short for baseband signalling as opposed to broadband, and the 5 stands for the maximum segment length of 500 metres (1,600 ft).[2]

Electromagnetic Spectrum

Basic limitations

• Attenuation• Delay Distortion• Noise

• Thermal/White Noise• Intermodulation Noise• Crosstalk• Echo• Impulse Noise

Design Factors for Transmission Media

• Bandwidth: All other factors remaining constant, the greater the band-width of a signal, the higher the data rate that can be achieved.

• Transmission impairments. Limit the distance a signal can travel.• Interference: Competing signals in overlapping frequency bands

can distort or wipe out a signal.• Number of receivers: Each attachment introduces some

attenuation and distortion, limiting distance and/or data rate.

Transmission media

Guided Unguided

Twisted-paircable

Coaxialcable

Fiber-opticcable

• Conducted or guided media• use a conductor such as a wire or a fiber optic cable to

move the signal from sender to receiver• Wireless or unguided media

• use radio waves of different frequencies and do not need a wire or cable conductor to transmit signals

Classes of Transmission Media

Wire Conductors• Wire types: single conductor, twisted pair, & shielded multiconductor bundles.• Reasonable cost.• Relatively low bandwidth, however, recent LAN speeds in the 100 Mbps range

have been achieved.• Susceptible to external interference.• Shielding can reduce external interference.• Can transmit both analog and digital signals. Amplifier required every 5 to 6 km

for analog signals. For digital signals, repeaters required every 2 to 3 km.

Difference between Amplifier and Repeater1. Amplifier is used to magnify a signal, whereas repeater is used to receive and retransmit a signal with a power gain.2. Repeater has an amplifier as a part of it.3. Sometimes, amplifiers introduce some noise to the signal, whereas repeaters contain noise eliminating parts.

Wired - Twisted Pair• The oldest, least expensive, and most commonly used

media• Pair of insulated wires twisted together to reduce

susceptibility to interference : ex) capacitive coupling, crosstalk

• Skin effect at high frequency• Up to 250 kHz analog and few Mbps digital signaling ( for

long-distance point-to-point signaling)• Need repeater every 2-3 km (digital), and amplifier every

5-6 km (analog)

• Consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs

• Often used at customer facilities and also over distances to carry voice as well as data communications• Low frequency transmission medium• Telephone (subscriber loop: between house and local exchange)• High-speed (10 - 100 Mbps) LAN :

• token ring, fast - Ethernet

Twisted Pair

Types of Twisted Pair

• STP (shielded twisted pair)• the pair is wrapped with metallic foil or braid to insulate

the pair from electromagnetic interference• UTP (unshielded twisted pair)

• each wire is insulated with plastic wrap, but the pair is covered in an outer covering

Ratings of Twisted Pair• Category 3

• UTP cables and associated connecting hardware whose transmission characteristics are specified up to 16 MHZ.

• data rates of up to 16mbps are achievable• Category 4

• UTP cables and associated connecting hardware whose transmission characteristics are specified up to 20 MHz.

• Category 5• UTP cables and associated connecting hardware whose transmission characteristics are

specified up to 100 MHz.• data rates of up to 100Mbps are achievable• more expensive, but better performance

• Category 5 enhanced, Cat 6, cat 7• Fast and giga-ethernet

• STP• More expensive, harder to work with

Twisted Pair Advantages

• Advantages• Inexpensive and readily available• Flexible and light weight • Easy to work with and install

• Disvantages • Susceptibility to interference and noise• Attenuation problem

• For analog, repeaters needed every 5-6km• For digital, repeaters needed every 2-3km

• Relatively low bandwidth (MHz)

Twisted-Pair Cable

Unshielded Twisted-Pair Cable

Shielded Twisted-Pair Cable

Wired Transmission Media

• Coaxial Cable • Most flexible medium => LANs, Cable TV, Long-distance telephones, VCR-to-TV connections • Noise immunity is good• Very high channel capacity => few 100 MHz / few 100 Mbps • Need repeater/amplifier every few kilometer or so (about the same as with

twisted pair)• Has an inner conductor surrounded by an outer mesh of wires• Both conductors share a common center axial, hence the term “co-

axial”

Coaxial cable• Signal and ground wire

• Solid center conductor running coaxially inside a solid (usually braided) outer circular conductor.

• Center conductor is shielded from external interference signals.

Properties of coaxial cable• Better shielding allows for longer cables and higher transfer rates.• 100 m cables

• 1 to 2 Gbps feasible (modulation used)• 10 Mbps typical

• Higher bandwidth• 400 to 600Mhz• up to 10,800 voice conversations

• Can be tapped easily: stations easily added (pros and cons, why?)• Much less susceptible to interference than twisted pair• Used for long haul routes by Phone Companies.

• Mostly replaced now by optical fiber.• High attenuation rate makes it expensive over long distance• Bulky • Baseband vs. broadband coax.

Wired Transmission Media

• Optical Fiber• Flexible, thin (few to few hundred m), very pure

glass/plastic fiber capable of conducting optical rays • Extremely high bandwidth : capable of 2 Gbps • Very high noise immunity, resistant to electromagnetic

interference • Does not radiate energy/cause interference• Very light• Need repeaters only 10’s or 100 km apart• Very difficult to tap : Better security but multipoint not easy• Require a light source with injection laser diode (ILD) or

light-emitting diodes (LED)

Wired Transmission Media• Optical Fiber (Cont’d)

• Need optical-electrical interface (more expensive than electrical interface)

Wired Transmission Media

• Principle of optical fiber transmission: Based on the principle of total internal reflection

• If >, medium B (water) has a higher optical density than medium A (air)• If is less than a certain critical angle, there is no refracted light i.e., all the

light is reflected. This is what makes fiber optics work.

Optical Fiber

Fiber optics & Physics 101• Refractive indexmaterial =

(Speed of light in vacuum)/(Speed of light in material)• Light is bent as it passes through a surface where

the refractive index changes. • This bending depends on the angle and refractive

index. • Frequency does not change, but because it slows

down causing wave to bend.• In case of fiber optic media, refractive index of core

> refractive index of cladding thereby causing internal reflection.

core

cladding

plastic jacket glass or plasticcladding fiber core

Fiber Optic Layers

• consists of three concentric sections

Modes of fiber• Fiber consists of two parts: the glass core and glass cladding with a lower

refractive index.• Light propagates in 1 of 3 ways depending on the type and width of the core

material.• Multimode stepped index fiber

• Both core and cladding have different but uniform refractive index.• Relies on total internal reflection; Wide pulse width.

• Multimode graded index fiber• Core has variable refractive index (light bends as it moves away from core). • Narrow pulse width resulting in higher bit rate.

• Singlemode fiber (> 100 Mbs)• Width of core diameter equal to a single wavelenth.

Mode

Multimode Single mode

Step index Graded-index

Fiber Optic Types

• multimode step-index fiber• the reflective walls of the fiber move the light pulses to

the receiver• multimode graded-index fiber

• acts to refract the light toward the center of the fiber by variations in the density

• single mode fiber• the light is guided down the center of an extremely

narrow core

Types of optical fiber• Modes, bundles of light rays enter the fiber at a particular

angle• Single-mode

• Also known as mono-mode• Only one mode propagates through fiber• Higher bandwidth than multi-mode• Longer cable runs than multi-mode• Lasers generate light signals• Used for inter-building connectivity

Types of optical fiber• Multi-mode

• Multiple modes propagate through fiber• Different angles mean different distances to travel

• Transmissions arrive at different times• Modal dispersion

• LEDs as light source• Used for intra-building connectivity

fiber optic multimodestep-index

fiber optic multimodegraded-index

fiber optic single mode

Fiber Optic Signals

Optical Fiber Transmission Mode

Fiber Optic

Advantages• greater capacity (bandwidth Gbps)• smaller size and lighter weight• lower attenuation• immunity to environmental interference• highly secure due to tap difficulty and lack of signal radiationDisvantages• expensive over short distance• requires highly skilled installers• adding additional nodes is difficult

Fiber Channel Requirements• Full duplex links with 2 fibers/link• 100 Mbps – 800 Mbps• Distances up to 10 km• Small connectors• high-capacity• Broad availability• Support for multiple cost/performance levels• Support for multiple existing interface command sets

Components of an optical transmission system

• 3 components1. Light source2. Transmission medium3. The detector

• Light means a 1 bit, no light means a 0 bit.• Transmitter LED or injection laser diode.• Detector (photodiode or photo transistor) generates an

electrical pulse when light falls on it.• Unidirectional data transmission system.

• Electrical signal to light signal and back again.

Fiber cables

• Multimode: diameter of core is ~50 microns.• About the same as a human hair.

• Single mode: diameter of core 8-10 microns.• They can be connnected by connectors, or by splicing, or by fusion.

Fiber vs. copper• Fiber (pros)

• Higher bandwidth, • Lower attenuation, • Immune to electromagnetic noise and corrosive chemicals,• Thin and lightweight,• Security (does not leak light, difficult to tap).

• Fiber (cons)• Not many skilled “fiber engineers,”• Inherently unidirectional,• Fiber interfaces are expensive.

Physical Media

• physical link: transmitted data bit propagates across link

• guided media: • signals propagate in solid

media: copper, fiber• unguided media:

• signals propagate freely e.g., radio

Twisted Pair (TP)• two insulated copper wires

• Category 3: traditional phone wires, 10 Mbps ethernet

• Category 5 TP: 100Mbps ethernet

Physical Media: coax, fiber

Coaxial cable:• wire (signal carrier) within a

wire (shield)• baseband: single channel on

cable• broadband: multiple channel

on cable• bidirectional• common use in 10Mbs

Ethernet

Fiber optic cable:• glass fiber carrying light pulses• high-speed operation:

• 100Mbps Ethernet• high-speed point-to-point

transmission (e.g., 5 Gbps)• low error rate

Fiber vs Satellite

Choosing the Right Transmission Media

• Areas of high EMI or RFI• Corners and small spaces• Distance• Security• Existing infrastructure• Growth

Media: Selection Criteria

• Cost (Initial, Expansion, & Maintenance)• Speed (Data Rate & Response Time)• Availability• Expandability• Error Rates• Security• Distance (Geography & Number of Sites)• Environment• Application-Specific Constraints• Maintenance

Media: Selection Criteria

Media Advantages DisadvantagesTwisted PairWire

Inexpensive.Easy to install.Large installed base.

Fairly low bandwidth.Does not handle highfrequencies well.

Coaxial Cable Fairly inexpensive.Fairly high bandwidth.

Bulky and somewhatinflexible.

Fiber OpticCable

Unaffected by “noise.”Very high bandwidth.

Expensive to install.

Satellite No line of sight needed.No cabling required.High bandwidth.

Leased channels.Initial equipment cost.Long delays (GEOS).

TerrestrialMicrowave

No cabling required.High bandwidth.

Line of sight required.Towers/repeaters needed.Initial equipment cost.

From Signals to PacketsAnalog Signal

“Digital” Signal

Bit Stream 0 0 1 0 1 1 1 0 0 0 1

Packets0100010101011100101010101011101110000001111010101110101010101101011010111001

Header/Body Header/Body Header/Body

ReceiverSenderPacket

Transmission

Chapter Summary

• Information can be transmitted via analog or digitally• Both signals suffer attenuation• Costs depend on many factors• Length of a network segment is limited due to attenuation• Coaxial cable consists of central copper core surrounded by an insulator

and a sheath• To make correct media transmission choices, consider, throughput,

cabling, noise resistance, security/flexibility, and plans for growth

Chapter Summary

• Twisted-pair cable consists of color-coded pairs of insulated copper wires, twisted around each other and encased in plastic coating

• The more twists per inch in a pair of wires, the more resistant to noise• STP cable consists of twisted pair wires individually insulated and

surrounded by a shielding• UTP cabling consists of one or more insulated wire pairs encased in a

plastic sheath• UTP comes in a variety of specifications• Fiber-optic cable contains one or several glass fibers in its core• On today’s networks, fiber is used primarily as backbone cable